The capacity to accurately measure bone and joint kinematics is key to understanding and quantifying the movement of the body, and understanding how such movement is altered by injury, disease, and treatment. Precise measurement of such kinematics is technically challenging. The most widespread and non-invasive method of determining joint kinematics is skin-mounted, optically reflective markers. Numerous models for marker placement about the foot have been created and are in use. Quantifying the error of these models which arises from skin motion artifact is a challenging task without resorting to invasive methods. Even with these potential problems, optical marker tracking has proved to be a powerful tool. Therefore the goal of this pilot study is to utilize another tool, biplane fluoroscopy, to validate an optical marker pattern or model - in this case, of the foot. Biplane fluoroscopy is a method of measuring the actual movement of bones using 3D geometry and projected 2D views. The validation is possible because biplane fluoroscopy is known to possess two orders of magnitude greater accuracy and precision than is commonly found with optical systems - it is however invasive due to the X-ray exposure, and thus not suited to widespread or regular use as a diagnostic aid. [In addition to evaluating overall errors, using biplane fluoroscopy will allow us to develop error models for each bone during gait. This will identify when, in terms of % stance gait, markers are particularly prone to error.] To this end the specific aims of this work are: SA1 - To determine [and model] the overall kinematic error of skin-mounted marker systems when compared to biplane fluoroscopy. This will better quantify how the differences in system resolutions fully impact kinematic results. To accomplish this, 15 research subjects will receive CT scans to quantify their bony geometry (this is a necessary step for biplane fluoroscopy), and then be fitted with optical skin mounted markers on their foot. These skin mounted markers will have been modified with the inclusion of a small metal bead within them. Subjects will then walk through an imaging field (capturing both optical data and fluoroscopic data) such that the reflective markers, the beads within them, and the bones themselves will be visible. By calculating and comparing kinematic data from the optical markers to the bone motions as determined from biplane fluoroscopy, we can determine optical system error and region (of the foot) error. SA2 - To quantify specific sources [and locations] of skin tissue artifact (STA) and hardware error associated with skin-mounted kinematic markers. Measuring specific marker and regional errors will allow us to identify exactly how well or poorly a particular model predicts kinematics. This is a multi-facet comparison of optical skin markers to fluoroscopic skin markers and bone-based tracking. This Aim will specifically evaluate individual marker and region/cluster errors [, to determine where the weakness of the model lies.] This pilot study will lay the groundwork for a more comprehensive investigation into ideal marker locations, segment choice and an understanding of such parameters as the effect of subject age and skin elasticity to skin motion artifact. The results of this work will directly benefit an ongoing skin-mounted marker based kinematic study of Veterans, and provide insight to a part of the research field currently ill-addressed. Specifically, the results obtained from this study will be used tailor marker models to more accurately perform research on populations that are receiving various treatments for ankle arthritis, such as arthroplasty (joint replacement) and arthodesis (joint fusion).